1. Field of the Invention
The invention relates to a process for biological purification of waste water by the activated sludge method, whereby the waste water is passed continuously through one or more treatment zones and a clarification zone for division of the treated waste water into a water fraction and a sludge fraction, and whereby a portion of the sludge fraction is recirculated and mixed with non-treated waste water.
2. The Prior Art
It is known that the efficiency of a biological purification of waste water by the activated sludge method increases with increasing amounts of sludge in the treatment zones, and that in order to obtain high sludge concentrations it has been necessary to employ high sludge recirculation ratios, i.e., high ratios of sludge, which is recirculated from the clarification zone to the first treatment zone, to the amount of non-treated waste water.
However, the large amount of sludge causes problems in case of increased supply of water, e.g., in case of heavy showers. In such a case the amount of sludge in the clarification tank will start increasing due to overload, which ultimately entails that the sludge is entrained out into the recipient with resultant pollution hereof.
It has therefore been attempted to find a way in which it is possible, perhaps temporarily in case of peak loads, to reduce the load on the clarification tank, so that the situation described above is avoided.
Bent Tholander has described a method which is based on a basic module formed of a continuously aerated tank and two side tanks with alternating function as aeration tanks or clarification tanks, where the waste water is always supplied to the centre tank. If two such modules are connected, a system is obtained which in case of particularly heavy showers can be adjusted, so that two clarification tanks are in operation.
PCT-application No. PCT/DK95/00309 (WO 96/02468) discloses a method of obtaining high sludge concentrations without using high sludge recirculation ratios by maintaining for a period such conditions in one or more treatment zones that sludge precipitation occurs herein, and by passing, in a succeeding period, a mixture of non-purified waste water and recirculated sludge to the zone which in the preceding period served as sludge precipitation zone.
By using the above mentioned method it has been possible to increase the sludge concentration in the treatment zone from 4 to about 5.9 kg/m.sup.3 and to reduce the sludge recirculation ratio from 1.0 to 0.5, which has resulted in a reduction of the load on the clarification tank from about 8.000 kg/h to about 3.000 kg/h.
In practice, the method described above presupposes that at least two, and preferably four, separate tanks and a clarification tank be used for treating the waste water.
This entails comparatively high costs for the construction and operation of water purification plants in which the method described above can be used.
DE 3 601 669 discloses a method where purification of waste water takes place in one undivided tank with several treatment zones, all with stirring, characterized by different oxygen concentrations. The method is based on the presence of carrier particles which make it possible to mechanically separate the purified waste water by means of a sieve device.
However, use of such a carrier material is associated with difficulties in the form of providing and removing it.
U.S. Pat. No. 4,457,844 discloses a method for the purification of waste water where a portion of the waste water flow is passed into an oxidation tank with an internal clarification device with inlet in the rear end and openings for sludge withdrawal at the bottom. The purified waste water is returned to the non-purified waste water flow.
Plants operated by this method will, however, be restricted by the physical shape of the clarification device, and their capacity thus cannot be extended no matter whether the need might change. Besides, the clarification may turn out to be inadequate, and means have not been provided for succeeding, extra clarification.
EP 191 483 discloses a method for biological aerobic treatment of waste water where the aeration of the waste water is adjusted proportionally to the hydraulic load on the plant, and where a gravimetric separator mechanically separates off and automatically returns sludge.
However, there is no post-clarification of the purified waste water, and consequently no change in the clarification capacity in the plant.
EP 30 485 discloses a method for biological treatment of waste water which takes place in one single tank which is divided into a reaction zone at the bottom of the tank, and an overlying clarification zone. The non-purified waste water is mixed with a recirculated flow which is withdrawn at the bottom of the tank. The mixture is aerated by means of an external air lift device, and is then returned to the tank. Purified waste water is withdrawn via an overflow at the top of the tank.
This method only allows short residence time in the external air lift, for which reason there is risk that the aeration gets unsatisfactory. In addition, the construction of the tank offers no possibility of increasing the capacity in case of peak loads.
DE 35 00 340 discloses a method for biological purification of waste water, where the waste water is passed through two stages from which sludge is mechanically separated off and recirculated.
The problem with plants operated by this method is that, as is the case for several of the plants already mentioned, in case of peak loads the required flexibility is not available. For example, it is not possible to employ more aeration capacity, should need arise.